Recently, reduction of emission amount of carbon dioxide is strongly desired in view of environmental protection. In automotive industry, more expectations are focused on reduction of emission amount of carbon dioxide by introduction of an electric vehicle, a hybrid electric vehicle, a fuel cell vehicle, a hybrid fuel cell vehicle, or the like. Development of a secondary battery for motor drive, a key element for practical application thereof, is now intensively carrying out. As a secondary battery, a lithium ion secondary battery is noticed, which is capable of attaining high energy density and high output density. However, use of a plurality of secondary batteries by joining in series is required to be applied as a power source for motor drive for such various kinds of vehicles as described above, and to secure high output.
However, joining of batteries via joining members results in reduction of output caused by electric resistance at the joining members. In addition, batteries having joining members also have spatial disadvantage; namely, the joining members incur reduction of output density or energy density of the batteries.
As a method for solving this problem, a bipolar battery where a positive electrode and a negative electrode are arranged at the both sides of a collector has been developed.
Among these, a bipolar battery made by using a polymer solid electrolyte not containing a solution in an electrolyte layer has been proposed (see, for example, JP-A-2000-100471). According to this proposal, because of no containing of a solution (an electrolyte solution) in a battery, such a bipolar battery can be provided as having no risk of liquid leak out or gas generation, high reliability and in addition not requiring airtight seal in structural viewpoint. However, ion conductivity of a polymer solid electrolyte is lower as compared with a polymer gel electrolyte, and output density or energy density of a battery is not sufficient in usual use environment, and thus a practically applicable stage has not yet been attained up to now, and further improvement of ion conductivity is required.
On the other hand, a bipolar battery made by using a polymer electrolyte containing an electrolyte solution in an electrolyte layer has been proposed (see, for example, JP-A-2002-75455 and JP-A-11-204136). Because use of a polymer electrolyte containing an electrolyte solution, namely a polymer gel electrolyte, provides excellent ion conductivity, as well as sufficient output density or energy density of a battery, this battery is expected as a bipolar battery closest to a practical application stage.